An RF (Radio Frequency) communication system, designed for conventional dispatch operation, generally includes one or more base sites and a plurality of mobile and portable communication units. In order to serve the needs of a large number of users, the base site is generally equipped with multiple repeater units, where each repeater unit operates over one communication resource. As is well known, a repeater can receive inbound communications over one communication resource, and retransmit the communication using another resource. While the term communication resource is usually understood to refer to an RF channel, the term could also mean a pair of RF channels, in the case of full duplex operation, or TDM (Time Division Multiplex) slots, in the case of a TDM communication system.
Communication units belonging to different companies or organizations are often assigned to different communication resources (or channels) as a convenient method of separation. In cases where the system includes a large number of users and/or the number of communication resources is limited, mobile or portable units belonging to different organizations or companies must share the same communication resource. This is often accomplished through the use of a coded squelch scheme. Coded squelch, as is well known in the art, may be implemented as a nonaudibly transmitted digital or analog signal that uniquely identifies a particular user or group of users within the RF communication system.
Even though individual companies, organizations, or groups of users may be assigned to a particular channel or communication resource, it is often necessary to be able to monitor communications occuring on other communication resources. This requirement is particularly evident in the case of public safety users, such as police departments and fire departments, where units are often grouped by function, such as tactical, surveillance, paramedics, narcotics, etc.
The use of channel scan techniques affords communication system users the opportunity to monitor activity occuring on communication resources other than the one to which an individual unit has been assigned. In prior art channel scan methods, each mobile or portable unit may be programmed to sequentially monitor a predetermined list of communication resources, checking for activity. When activity is detected on a particular communication resource that is being scanned, the scan is halted and the communication unit remains tuned to the particular communication resource until the activity has ended, then resumes its scan operation.
The use of coded squelch systems, as discussed above, makes the channel scan process more complex. When coded squelch is in use, communications of potential interest may be further categorized according to predetermined coded squelch signals. So, in implementing channel scan, a mobile or portable unit will sequentially monitor predetermined communication resources, and upon detecting activity, the mobile or portable unit will determine if the coded squelch signal being transmitted identifies a communication unit or group of communication units of interest. Since it requires processing time to correctly identify a particular coded squelch signal, the use of coded squelch increases the time required for effective channel scan operation.
The use of prior art channel scan techniques creates no insurmountable problems except when a mobile or portable unit utilizing a channel scan function is engaged in conversation. It is generally considered important that channel scan continue even when a mobile or a portable unit is in the process of communicating with another unit so that critical communications occuring over other communication resources are not missed. In the case of a police agency, for example, it may be particularly critical that communication among other groups of police users be closely monitored.
One disturbing effect of conventional channel scan operation during conversation is interruption of receive audio in the current conversation. The scanning communication unit must at least temporarily leave the communication resource over which conversation is occurring, and sequentially monitor the other communication resources of interest. In communication systems where coded squelch is not implemented, the sequential monitoring operation creates only short bursts of noise in the communication unit's receive audio, coincident with a brief monitoring window. However, in coded squelch systems, the burst of noise may be quite lengthy, since identification of particular coded squelch signals requires a considerably longer time than mere identification of activity.
In some instances, most common perhaps in the case of police agencies, communications security is also an important factor. To satisfy this need for communication security, many of the conversations occurring over communication resources within the system will be encrypted to prevent unauthorized listeners from learning the details of police operations. The use of encryption severely complicates the channel scan process. It takes even longer to determine if an encrypted transmission has been encrypted with a particular key (a special code combination allowing a receiver to decrypt encrypted transmissions) than it does to decode a coded squelch signal. This creates an unacceptable interruption of any conversation in which the scanning communication unit is involved. For most encrypted systems, the action of leaving the communication resource over which conversation is occurring, in order to monitor other communication resources, would result in the loss of synchronization information and an unacceptable gap in received information.
Since channel scan is an important and desirable feature of RF communication systems, and the ability to monitor higher priority communications, even while engaged in conversation, may be an essential feature, particularly for police agencies and other public safety users, a need arises for a method of implementing channel scan without causing unacceptable interruptions in the receive audio of conversations in progress during channel scan operation.